At about noon I start gazing around for suspects. Soon it will be lunchtime. The Piazza di Spagna is crowded, the stairs of Trinità dei Monti are so full it’s hard to walk, and Bernini’s Fontana della Barcaccia is being photographed from every angle. But amid this chaos my role is simple: to make sure people don’t start eating their sandwiches or, worse, set up full picnics right on top of the monuments here in Rome.

As soon as anyone does – and it happens every day without exception – I run to stop them. It’s my duty as a municipal police officer to approach offenders and inform them that as of October a new law prohibits people from eating near any monument in Rome. Trespassers can be fined anything between €25 and €500, but since I am a nice guy I never demand more than €50. More would be too much, as it’s preferable that people pay on the spot.

Every time my colleagues and I catch someone we ask the same questions: “Remember when you went to the Vatican, how you couldn’t dress as you pleased? Weren’t the rules long trousers, no bare shoulders and no snacks? Why should it be different here?” At this point, there are really only two possible outcomes: the sympathetic reaction or the angry one.

Tourists and elderly folk usually fall within the first category. Some weeks ago I caught an American sitting comfortably at the Pantheon eating ice cream. When I explained what was going on, and why, he started shouting, “Yeah! Yeah! You’re completely right! How could I have not thought about it myself? It’s such a logical thing!”

But the second category, the angry one, is not as nice; quite the opposite. These are mainly youngsters and Italians. Once I came across a family from Naples sitting on the stairs of Piazza di Spagna with plates, forks, knives, olive oil and bowls full of pasta. When I rushed to stop them, they were head down and about to dig in. It’s unacceptable and indecent. Picnics are fine, but not in the centre of Rome where every corner is like an open-air museum. A classic angry reaction is, “Oh yeah? Where is it written? How should I know? This is so unfair. I’m not paying.” Thankfully, at least in Piazza di Spagna the municipality has erected a sign clearly stating the rules – but there aren’t any at the Coliseum, in Piazza Navona or by the Pantheon. Discussions with offenders can be long.

A lot of people call this law, which the Mayor of Rome pushed for, the “anti-sandwich ordinance”. That’s partly true, but it’s a little more complicated. First of all, it only applies to certain areas of the city – mainly in the very centre of town close to the most important monuments. Second, the law only forbids “halting” while eating on the grounds close to these monuments. So if someone is just passing by with a piece of pizza in his mouth it’s unlikely I would stop him. The real difficulty is that teenagers eating ice cream will inevitably drop dirty napkins next to where they were sitting, couples drinking wine might leave the bottle behind, groups of tourists could start eating and drop stuff from their panini. Yes, these are little things, but they quickly add up. Think of a cola drink: if it spills on marble, it permanently damages it. The city employs a specialist team just to clean the stairs of Piazza di Spagna – I don’t know how much that costs, but this law will certainly make their lives easier.

It might sound strange, but I take every opportunity to convince people that the decision to bring in this law, and to enforce it, is not so odd. Without exaggerating, I am happy to be helping to preserve the monuments of the eternal city.